There is substantial evidence to indicate that the partially reduced oxygen metabolites (oxidants) that are generated in the ischemic-reperfused heart are agents of injury to the myocardium, and that inactivations of NaK-ATPase (i.e., the sodium pump) and several other sarcolemmal ion carriers are early events in the oxidantinduced damage to the myocyte. Therefore, the long-range goals of our studies are (a) to characterize the mechanisms of oxidant interactions with these ion carriers; (b) to define the physiological mechanisms that regulate oxidant interactions with these sarcolemmal proteins; and (c) to develop pharmacologic approaches for the protection of sarcolemmal function from oxidant-induced damage. Our initial studies toward these goals have revealed that in some tissues there are two populations of NaK-ATPase with different sensitivities to oxidants, and that the catalytic properties of the two populations differ. These findings, coupled with the knowledge that different isoforms of NaK-ATPase have different digitalis sensitivities, prompt the proposal of the following specific aims: 1. To use cardiac tissues and preparations in which the existence of mixtures of different isoforms of NaK-ATPase is established to test the hypothesis that different isoforms have different oxidant sensitivities. 2. To use sealed cardiac sarcolemmal vesicles containing different isoforms, and characterize the pump-mediated ion fluxes. that are mediated by different isoforms in order to assess the pathophysiological significance of the different oxidant sensitivities of the different pump populations. 3. To use purified preparations of NaK-ATPase in order to determine the mechanism and the structural bases of oxidant-induced damage to the subunits of the pump. 4. To explore the effects of oxidants on each of three isoforms of NaK-ATPase that is expressed-in host cells containing no endogenous NaK-ATPase. Also, in order to test the hypothesis that intracellular superoxide dismutase has a defensive role in oxidant-induced damage to sarcolemmal function; we propose to alter the cardiac levels of this enzyme in copper deficient rats, and assess the oxidant sensitivities of several sarcolemmal ion carriers in intact cardiac myocytes isolated from these animals.